Technical Field
The present invention relates to a friction stir welding tool comprising a composite of a tungsten-rhenium alloy and hafnium carbide particles, wherein the tungsten-rhenium alloy has a crystallite size of no more than 100 nm.
Description of the Related Art
The “background” description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description which may not otherwise qualify as prior art at the time of filing, are neither expressly or impliedly admitted as prior art against the present invention.
Friction Stir Welding (FSW) process was invented by The Welding Institute (TWI) in 1991. See W. Konig and A. Neises: ‘Wear mechanisms of ultrahard, nonmetallic cutting materials’, Wear, 1993, 162, 12-21, incorporated herein by reference in its entirety. The process employs a spinning pin tool that produces frictional heat in the welding of the workpiece. The pin tool is pressed into contact with a seam to be welded. A typical FSW system is shown in FIG. 1. See M. Senmweldi and C. Metals, “ADVANCES IN TOOLING MATERIALS FOR FRICTION,” pp. 1-11, 1991, incorporated herein by reference in its entirety. The base metal heats up due to the rubbing of tool faces by visco-plastic dissipation of mechanical energy at high strain rates. See Y. Gao, and R. H. Wagoner, “A simplified model for heat generation during the uniaxial tensile test”, Metallurgical Transactions, 18A: 1001-1009,1987; H. C. Braga, and R. A. Barbosa, “Simulation of the increase in temperature due to adiabatic heating in hot deformation processes”, Proceedings of 47th Brazilian Association of Metallurgy and Materials (ABM). Annual Conference, pp. 441-457, ABM,1992; A. C. Nunes, E. L. Jr., Bernstein, and J. C. McClure, “A rotating plug model for friction stir welding”, 81st American Welding Society Annual Convention. Chicago, Ill.; Apr. 26-28, 2000, each incorporated herein by reference in their entirety. When the heat of the workpiece reaches about 80% of its melting point it becomes soft and easy to form joining.
FSW has been used for welding low melting point materials such as aluminum Microstructural examination of aluminum alloys joined by FSW exhibits several zones in the weld or bead. These are Stir Zone (SZ), Thermo-Mechanical Heat Affected Zone (TMAZ) around the nugget and Heat Affected Zone (HAZ). See L. E. Murr, G. Liu, and J. C. McClure, “A TEM study of precipitation and related microstructures in friction stir welded 6061 aluminum” Journal of Materials Science, 33: 1243-1251. 1998; C. G. Rhodes, M. W. Mahoney, W. H. Bingel, R. A. Spurling. and C. C. Bampton, “Effects of friction stir welding on microstructure of 7075 aluminum” Scripta Materiala, 36(1): 69-75, 1997; M. W. Mahoney, C. G. Rhodes, J. G. Flintoff, W. H. Bingel, and R. A. Spurling, “Properties of friction stir welded 7075 T651 aluminum” Metallurgical and Materials Transactions 29A: 1955-1964, 1998, each incorporated herein by reference in their entirety. Stir zone experiences the highest strain rates and consequently higher temperatures. See T. Weinberger, N. Enzinger, and H. Cerjak. “Microstructural and mechanical characterisation of friction stir welded 15-5PH steel” Sci. Technol. Weld. Join, vol. 3, no. 14, pp. 210-215, 2009, incorporated herein by reference in its entirety. These zones are related to thermomechanical cycle during FSW of aluminum metal and alloys.
Although the FSW process has initially been developed for joining non-ferrous materials such as aluminum, by using suitable tool materials the use of the process has been extended to harder and higher melting point materials such as steels, titanium alloys and copper. Recently, a considerable attention has been given to FSW of high melting temperature alloys such as steel, due to the process advantages over conventional welding methods, which avoids many problems of fusion welding like porosity, cracking, and solidification. See T. Weinberger, N. Enzinger, and H. Cerjak, “Microstructural and mechanical characterisation of friction stir welded 15-5PH steel” Sci. Technol. Weld. Join, vol. 3, no. 14, pp. 210-215, 2009, incorporated herein by reference in its entirety.
Finding a high strength material as the friction stir welding tool is key to the success of the process. Recent efforts have been dedicated to produce cost effective and reusable tools for Friction Stir Welding (FSW) steel and hard alloys; however, most of these efforts need improvement in tool material development aspects. See S. Park, Y. Sato, H. Kokawa, K. Okamoto, S. Hirano and M. Inagaki: ‘Boride formation induced by pcBN tool wear in friction-stir-welded stainless steels’, Metall. Mater. Trans. A, 2009 40A, (3), 625-636: P. L. Raffo, “Yielding and fracture in tungsten and tungsten rhenium alloys” J. Less Common Met., vol. 2, no. 17, pp. 133-149, 1969; R. Rai, H. K. D. H. Bhadeshia, and T. Debroy, “Review: friction stir welding tools,” Sci. Technol., vol. 16, no. 4, pp. 325-342, 2011, each incorporated herein by reference in their entirety. FSW of these hard alloys put stringent conditions for the tool material as tool will be exposed to harsh condition during the process.
E. Y. Ivanov et al. investigated the synthesis of nanocrystalline tungsten-rhenium alloy by mechanical alloying. See E. Y. Ivanov, C. Suryanarayana, and B. D. Bryskin, “Synthesis of a nanocrystalline W-25 wt % Re alloy by mechanical alloying,” vol. 251, pp. 255-261, 1998, incorporated herein by reference in its entirety. They found that mechanical alloying of a W-25 wt. % a Re powder mixture for 14hr in a high energy mill led to the development of nanocrystalline W—Re alloy. Jonathan et al. investigated the effect of temperature and holding time on the relative density of W-25% Re mixture during spark plasma sintering and it was found that with the increase of temperature and hold time, the relative density decreases as shown in FIG. 8. See Jonathan A. Webb Indrajit Charit Cory Sparks Darryl P. Butt Megan Frary Mark Carroll, “SPS Fabrication of Tungsten-Rhenium Alloys in Support of NTR” Fuels Development, Nuclear and Emerging Technologies for Space 2011, INL/CON-10-20354, incorporated herein by reference in its entirety. This is attributed to the diffusion of carbon from graphite dies.
M. A Yar et al. synthesized Nano-crystalline W-1% Y2O3 powder by a modified solution chemical reaction of ammonium paratungstate (APT) and yttrium nitrate. See M. a. Yar, S. Wahlberg, H. Bergqvist, H. G. Salem, M. Johnsson, and M. Muhammed, “Spark plasma sintering of tungsten-yttrium oxide composites from chemically synthesized nanopowders and microstructural characterization,” J. Nucl. Mater., vol. 412, no. 2, pp. 227-232, May 2011, incorporated herein by reference in its entirety. Spark plasma sintering (SPS) was used to consolidate the powder at 1100 and 1200° C. for various holding times. It was found that dispersion of yttrium oxide enhanced the sinterability of W powder as compared to lanthanum oxide. Park et al. synthesized dense, ultrafine WC-10 wt. % Co tool materials by SPS and shaped the tool to perform FSW on steel. See H. Park, H. Youn, J. Ryu, H. Son, H. Bang, and I. Shon, “Fabrication and mechanical properties of WC-10 wt. % Co hard materials for a friction stir welding tool application by a spark plasma sintering process,” vol. 13, no. 6, pp, 705-712, 2012, incorporated herein by reference in its entirety. They also investigated the mechanical and microstructural investigation of the tool as well as steel. Luo et al. investigated the mechanical properties of W-3.6Re-.26HfC composite at 1700-2980K and found that HfC play very important role in strengthening the alloy up to 2960 K due its outstanding thermal stability at very high temperature. See Luo, K. S. Shin, and D. L. Jacobson, “Hafnium carbide strengthening in a tungsten-rhenium matrix at ultrahigh temperatures,” Acta Metall. Mater., vol. 40, no. 9, pp. 2225-2232, September 1992, incorporated herein by reference in its entirety. Mingqui et al. studied the growth behavior of HfC dispersed in the W—Re matrix and investigated the effect of its dispersion on the strength of the alloy at temperature above 2200K and they found that from 2200 K to 2600K there was little growth of HfC with very slow growth rate. See M. Liu and J. Cowley, “Hafnium carbide growth behavior and its relationship to the dispersion hardening in tungsten at high temperatures,” Mater. Sci. Eng. A, vol. 160, no. 2, pp. 159-167, February 1993, incorporated herein by reference in its entirety. Rapid growth occurs after 2600K due to enhanced diffusion along the grain boundaries. John et al. studied the high temperature creep behavior of tungsten-4 wt % rhenium-0.32 wt % hafnium carbide at temperatures ranging from 2200 to 2400 K at 40-70 MPa. See J. J. Park, “Creep strength of a tungsten-rhenium-hafnium carbide alloy from 2200 to 2400 K,” Mater. Sci. Eng. A, vol. 265, no. 1-2, pp. 174-178, June 1999, incorporated herein by reference in its entirety. The stress exponent for secondary stage creep was 5.2. Activation energy for this stage was found to be 594 kJ/mol. Rea et al. consolidated the W-1.3 wt % HfC by hot isostatic pressing. See K. E. Rea, V. Viswanathan, a. Kruize, J. T. M. De Hosson, S. O'Dell, T. McKechnie, S. Rajagopalan, R. Vaidyanathan, and S. Seal, “Structure and property evaluation of a vacuum plasma sprayed nanostructured tungsten-hafnium carbide bulk composite,” Mater. Sci. Eng. A, vol. 477, no. 1-2, pp. 350-357, March 2008, each incorporated herein by reference in its entirety. High resolution transmission electron microscopy (HRTEM) of the consolidated sample is indicated the uniform dispersion of nanosize HfC in the tungsten matrix.
Anhua et al. presented the effects of rhenium concentration on the strength properties of the W—Re—ThO2 alloys at high temperatures and the yield strength of the alloys decreased with increasing temperature due to strengthening effect of Re in the system. See A. Luo, K. Shin, and D. Jacobson, “High temperature tensile properties of W—Re—ThO2 alloys,” Mater. Sci. Eng. A, 1991, incorporated herein by reference in its entirety. Liu et al. investigated the effect of micro-size (0.2% Zr) alloying and nano-sized (1% Y2O3) oxide dispersion in tungsten and then sintered by SPS. Oxygen at W grain boundaries reacts with Zr to form zirconia. See R. Liu, Z. M. Xie, T. Hao, Y. Zhou, X. P. Wang, Q. F. Fang, and C. S. Liu, “Fabricating high performance tungsten alloys through zirconium micro-alloying and nano-sized yttria dispersion strengthening,” vol. 451, pp. 35-39, 2014, incorporated herein by reference in its entirety.
S. K. Rakhunathan et al. studied the high energy and high rate consolidation of W and W based composites. See S. K. Raghunathan, C. Persad, and D. L. Bourell, “High-energy, High-rate Consolidation of Tungsten and Tungsten-based Composite Powders,” Mater. Sci., vol. 131, pp. 243-253, 1991, incorporated herein by reference in its entirety. The pure tungsten compact exhibited a ductile failure of the infiltered copper matrix. Z. Zak Fang et al. reviewed the synthesis, sintering, and mechanical properties of nanocrystalline cemented tungsten carbide. See Z. Z. Fang, X. Wang, T. Ryu, K. S. Hwang, and H. Y. Sohn, “Int. Journal of Refractory Metals & Hard Materials Synthesis, sintering, and mechanical properties of nanocrystalline cemented tungsten carbide—A review,” Int. J. Refract. Met. Hard Mater., vol. 27, no. 2, pp. 288-299, 2009, incorporated herein by reference in its entirety. They also discuss the effect of addition of grain growth inhibitor such as VC on the grain size. They reported that there was almost no grain growth up to 1100° C. and addition of VC inhibits the grain growth at higher temperatures.
David et al. studied the dislocation density as a result of implantation of ions on W—Re ions. See D. E. J. Armstrong and T. B. Britton, “Effect of dislocation density on improved radiation hardening resistance of nano-structured tungsten-rhenium,” Mater. Sci. Eng. A, vol. 611, pp. 388-393, August 2014, incorporated herein by reference in its entirety. Increase in hardness measured by nanoindentation was attributed to the interaction between irradiation loop and dislocations. Dongju et al. investigated the effect of milling and sintering on the phases of HfC—W composite. See D. Lee, M. A. Umer, H. J. Ryu, and S. H. Hong, “The effect of HfC content on mechanical properties HfC—W composites,” Int. J. Refract. Met. Hard Mater., vol. 44, pp. 49-53, May 2014, incorporated herein by reference in its entirety. This class of composite was spark plasma sintered at 1800° C. Ozherelyev et al. discussed the X-Ray Diffraction studies of Hf—W alloys. See V. V. Ozherelyev, a. I. Bocharov, a. V. Bondarev, and Y. V. Barmin, “X-ray diffraction study of atomic structure of Hf—W amorphous alloys,” Int. J. Refract. Met. Hard Mater., vol. 48, pp. 141-144, January 2015, incorporated herein by reference in its entirety. They found that with the increase of tungsten contents, positions of the peaks shift from right to left suggesting solid solution formation.
Shuaib et al. reported the results of friction stir welding of tube-tubesheet joints made of steel. See F. A. Al-Badour, N. Merah, A. N. Shuaib, and A. Bazoune, “Experimental Investigation of Friction Stir Seal Welding of Tube-Tubesheet” Journal of Pressure Vessel Technology, Vol. 137/011402, 2015, incorporated herein by reference in its entirety. Void defects were reported at the root of some welded regions. Larger voids were observed at the joints having holes without chamfers compared to those with holes with chamfers. Chung et al. conducted a study of FSW (WC tool) for high carbon steel below and above eutectoid temperature. See C. Y. Dong, F. Hidetoshi, N. Kazuhiro, and N. Kiyoshi, “Friction stir welding of high carbon tool steel (SK85) below Eutecoid temperature” Transactions of JWRI, vol. 38, no. 1. pp. 37-41, 2009, incorporated herein by reference in its entirety. These authors reported the presence of a mixture of pearlite and cementite structure present below A1 (A1 temperature below which there will be no phase transformation) whereas all other conditions show martensite plus pearlite structure. In this investigation it was found that below A1 with 100 mm/min welding speed and 100 rpm rotation speed, the microstructure is totally pearlite and cementite whereas when conditions were changed to 200 mm/min and 400 rpm the microstructures at locations of joints were different at the top 65% was the martensite which decreases to 20% at the bottom.
B. W Ahn et al. investigated the FSW of 409L SS by using a silicon nitride tool. See B. W. Ahn, D. H. Choi, D. J. Kim, and S. B. Jung, “Microstructures and properties of friction stir welded 409L stainless steel using a Si 3 N 4 tool,” Mater. Sci. Eng. A, vol. 532, pp. 476-479, 2012, incorporated herein by reference in its entirety. The base metal (BM) has hot rolled ferrite grain structure and stir zone (SZ) had an equiaxed ferrite grain structure with a diameter of approximately 50 μm. The equiaxed grain size was formed due dynamic recrystallization. Meshram et al. investigated the mechanical behavior of friction stir welding of stainless steel performed by PCBN pin tool. See M. P. Meshram, B. K. Kodli, and S. R. Dey, “Friction Stir Welding of Austenitic Stainless Steel by PCBN Tool and its Joint Analyses,” Procedia Mater. Sci., vol. 6, no. Icmpc, pp. 135-139, 2014, incorporated herein by reference in its entirety. The base metal was found to have 608 MPa UTS whereas Friction stir welded sample showed 630 MPa. The weld behaved almost similar to base metal. Konkol et al. compared the Friction Stir Welding and Submerged Arc Welding of HSLA-65 Steel. See Konkol, “Comparison of Friction Stir Weldments and Submerged Arc Weldments in HSLA-65 Steel,” no. July, pp. 187-195, 2007, incorporated herein by reference in its entirety. The welding of the 3 m length of HSLA-65 with the refractory alloy tool was done successfully. The W—Re pin showed almost no wear or change in length at the completion of the weld.
Zafar et al. investigated the effect of friction stir welding parameters on the weld microstructure of mild steel using a W-25% Re pin tool. See Z. Iqbal, A. N. Shuaib, F. Al-badour, N. Merah, and A. Bazoune, “Microstructure and hardness of friction stir weld bead on steel plate using W-25% Re pin tool.,” Proc. ASME 2014 12th Bienn. Conf Eng. Syst. Des. Anal. ESDA Jun. 25-27, 2014, Copenhagen, Denmark, pp. 1-6, 2014, incorporated herein by reference in its entirety. The effect of tool travel speed on bead surface finish as well as bead width can be observed from FIG. 9 where the weld bead is superimposed on the load profiles of the pin tool. In the first 30 mm length of the bead, the tool was traveling at 15 mm/min zone (A). Toward the end of zone (A), two surface defects or discontinuities, marked by dashed circles, developed, which caused an increase in the magnitude of the axial welding force.
Buffa et al. investigated a quantitative analysis of the tool life in FSW of 3 mm thick Ti-6V-4Al titanium alloy sheets by using W-25% Re tool and found it successful in welding the alloy. See G. Buffa, L. Fratini, F. Micari, and L. Settineri, “On the Choice of Tool Material in Friction Stir Welding of Titanium Alloys,” vol. 40, 2012, incorporated herein by reference in its entirety. Lienert et al. studied the FSW for joining of mild steel by characterizing the process of friction stir welds on mild steel by using refractory tools. See T. J. Lienert, W. L. Stellwag, Jr., B. B. Grimmett, and R. W. Warke, “Friction Stir Welding Studies on Mild Steel” Welding Research, Supplement to the Welding Journal, January, pp. 1-9. 2003, incorporated herein by reference in its entirety. Park et al. synthesized dense, ultrafine WC-10 wt. % Co tool materials by SPS and shaped the tool to perform FSW on steel. This tool was used to friction stir weld the low carbon steel sheet of thickness 2 mm. No visible defects were present in the nugget of weld. Stir zone is the region where base material comes in direct contact with the tool and heat affected zone is the region where grain growth can be found.
Thompson et al. evaluated the diffusional wear of three different Tungsten base tool with same geometry for the FSW of steel and titanium alloys. See Thompson, B., Babu, S. S., 2010, Tool degradation characterization in the friction stir welding of hard metals, Welding Journal (Miami, Fla), 89 (12): 256s-261, incorporated herein by reference in its entirety. They found that Tungsten-Titanium workpiece cross diffusion was more rapid than Tungsten-steel cross diffusion. However, microstructural investigation showed that tungsten from the tool was diffused into the steel. They also reported that W—Re and W—Re—HfC tools showed minimal tool degradation. Barnes et al. investigated the effect of tool material on developed microstructure in FS welded HSLA-65 steel. See Barnes, S. J., Bhatti, A. R., Steuwer, A., Johnson, R., Altenkirch, J., & Withers, P. J. (2012). Friction stir welding in HSLA-65 steel: part I. Influence of weld speed and tool material on microstructural development. Metallurgical and Materials Transactions A, 43(7), 2342-2355, incorporated herein by reference in its entirety. The authors compared the performance of PNCB to W-25% Re, and they found that excessive level of abrasive wear occurred on the W—Re tool as compared with PNCB tool, and it found to be increasing with tool temperature.
FSW tool wear occurs when it passes through the workpiece. The reduction in yield strength of tool may happen due to high load application and elevated temperature generated during the FSW of harder metal and alloys such as steel and titanium alloys. Wear of the tool can be due to abrasion, adhesion or diffusion. Due to the involvement of high temperature, diffusional wear can play a vital role in the tool wear. From thermodynamic point of view, Ellingham diagram can help to find out relative ability of oxidation at elevated temperature for such metallic tools. In most of the cases, tool failures are related to pin rather than shoulder as pin has to face more resistance to motion when immersed in the workpiece. Moreover, the pin has lower load bearing capability when compared to shoulder part which results in higher torsional and bending stresses in the former. Steel and titanium alloys were recently friction stir welded by W—Re alloy tool as discussed earlier.
There are several varieties of tool materials available in the markets, which include: high carbon steels, high speed steels (HSS), and cemented carbides (WC)-based and ceramics, alumina-based, silicon nitride-based, sintered polycrystalline diamond), and sintered polycrystalline cubic boron nitride (PCBN). Diamond and cubic boron nitride (BN) are known as super hard materials due to their exceptional hardness. So we can say that going from carbon steels to diamond, the tool material shows an increase in wear resistance, hardness, plastic deformation resistance, and cost while the thermal shock resistance and ease of fabrication decrease.
Cubic boron nitride (cubic BN, Knoop hardness 4700 kgfmm−2) is not available in nature Synthesis of cubic BN requires the transformation of BN from hexagonal to cubic form at high temperature-high pressure. FIG. 5 compares important mechanical properties of friction and fusion welds with those of the parent metal. See K. Brookes, “There's more to hard materials than tungsten carbide alone,” Met. Powder Rep., vol. 66, no. 2, pp. 36-37, 39-45, 2011, incorporated herein by reference in its entirety. Diamond is the hardest materials (Knoop hardness 8000 kgfmm−2). The brittleness of some important tool materials such PCBN is a major issue that needs to be addressed.
Commercially pure tungsten (cp-W) is strong at higher temperatures but has poor toughness at room temperature. Pure tungsten exhibits high wear when utilized as a tool material for Friction Stir Welding of steels and titanium alloys. Exposure of cp-W to temperatures higher than 1473 K results in crystallization and brittleness when cooled to room temperature. Addition of Re to tungsten lowers the ductile to brittle transition temperature as a result of changing the Peierls stress for dislocation motion. This led to the development of tungsten rhenium alloys, with W-25 wt-% Re as a candidate material for FSW tools. Steels and titanium alloys are friction stir welded by W-25 wt-% Re tool. The weld microstructure can be affected due to interaction with the tool material. Wear of the tool will increase the cost of the tool if the tool has lower yield strength at elevated temperature. The tool material under investigation should have high strength, high thermal conductivity and low coefficient of thermal expansion at high temperature. The interaction of work piece with the tool is also important at high temperature. Pin tools made from PCBN and W based alloys have found to be suitable candidates for FSW of steel and titanium alloys.
PCBN (Polycrystalline Boron Nitride) and other ceramic tool materials such as Si3N4 are currently being used on commercial scale due to their high hardness and strength at elevated temperatures. However, processing PCBN involves a combination of very high temperatures and pressures. In addition, PCBN easily fail during the plunging stage due to its low toughness. Tool wear affects not only the tool life but also the weld characteristics. FSW of steels with PCBN involves boron and nitrogen pick-up from worn tool leaving the material susceptible to corrosion and pitting. Workpiece may be contaminated with Nitrogen. Nitrogen can also react with oxygen to make detrimental oxides. PCBN has high thermal conductivity (100-250 W m−1 K−1) which results in higher heat loss and lower workpiece temperatures.
Synthesis of the tool materials plays an important role in gauging the performance of the tool under severe conditions of friction stir welding of steel. Table 1 provides a comparison of mechanical alloying and sintering parameters for different tungsten base composites mainly consolidated by spark plasma techniques whereas Table 2 shows some other techniques used for the consolidation of tungsten base alloys and composites.
TABLE 1List of tungsten base materials synthesized by various techniquesSinterTemptimeSystem(° C.)(min)TechniqueRef.Pure tungsten1800 0-15PlasmaK. Cho, L. Kecskes, R. Dowding, B. Schuster, Q. Wei, and R. Z. Valiev,“Nanocrystalline and Ultra-Fine Grained Tungsten for Kinetic EnergyPenetrator and Warhead Liner Applications,” Mater. Res., no. June, 2007W—25Re2400180 Cold pressE. Y. Ivanov, C. Suryanarayana, and B. D. Bryskin, “Synthesis of ananocrystalline W— 25 wt. % Re alloy by mechanical alloying,” vol. 251,pp. 255-261, 1998W—Cu———Jonathan A. Webb Indrajit Charit Cory Sparks Darryl P. Butt MeganFrary Mark Carroll, “SPS Fabrication of Tungsten-Rhenium Alloys inSupport of NTR” Fuels Development, Nuclear and EmergingTechnologies for Space 2011, INL/CON-10-20354W—7Ni—0.1Y2O3150030Cold pressS. N. Alam, “Synthesis and characterization of W—Cu nanocompositesdeveloped by mechanical alloying,” Mater. Sci. Eng. A, vol. 433, no. 1-2,pp. 161-168, October 2006W—B4C1700—Cold pressF. Jing-lian, L. Tao, C. Hui-chao, and W. Deng-long, “Preparation of finegrain tungsten heavy alloy with high properties by mechanical alloyingand yttrium oxide addition,” J. Mater. Process. Technol., vol. 208, no.1-3, pp. 463-469, November 2008W—1%Y2O312003-5SPSN. Ünal, “Mechanical means help make better tungsten matrixcomposites,” Met. Powder Rep., vol. 63, no. 10, pp. 28-33, November 2008W—5Y2O31700 3SPSM. a. Yar, S. Wahlberg, H. Bergqvist, H. G. Salem, M. Johnsson, and M.Muhammed, “Spark plasma sintering of tungsten-yttrium oxidecomposites from chemically synthesized nanopowders and microstructuralcharacterization,” J. Nucl. Mater., vol. 412, no. 2, pp. 227-232, May 2011W—30 vol % HfC1850—SPSY. Kim, K. H. Lee, E.-P. Kim, D.-I. Cheong, and S. H. Hong,“Fabrication of high temperature oxides dispersion strengthened tungstencomposites by spark plasma sintering process,” Int. J. Refract. Met. HardMater., vol. 27, no. 5, pp. 842-846, September 2009WC—10 wt % Co120012SPSJ. Lee, J.-H. Kim, and S. Kang, “Advanced W—HfC Cermet using In-SituPowder and Spark Plasma Sintering,” J. Alloys Compd., October 2012
TABLE 2List of tungsten base composites synthesized by various techniquesSinterTemptimeSystem(° C.)(min)TechniqueRef.W—3.6Re—0.26HfC——Arc meltedH. Park, H. Youn, J. Ryu, H. Son, H. Bang, and I. Shon,“Fabrication and mechanical properties of WC-10 wt. % Co hardmaterials for a friction stir welding tool application by a sparkplasma sintering process,” vol. 13, no. 6, pp. 705-712, 2012W—3.6Re—0.35HfC——Arc meltedLuo, K. S. Shin, and D. L. Jacobson, “Hafnium carbidestrengthening in a tungsten-rhenium matrix at ultrahightemperatures,” Acta Metall. Mater., vol. 40, no. 9, pp. 2225-2232,September 1992W—4Re—0.32HfC——Arc meltedM. Liu and J. Cowley, “Hafnium carbide growth behavior and itsrelationship to the dispersion hardening in tungsten at hightemperatures,” Mater. Sci. Eng. A, vol. 160, no. 2, pp. 159-167,February 1993W—1.5HfC1800240HIPJ. J. Park, “Creep strength of a tungsten-rhenium-hafnium carbidealloy from 2200 to 2400 K,” Mater. Sci. Eng. A, vol. 265, no. 1-2,pp. 174-178, June 199990W—7Ni—3Fe150030MWK. E. Rea, V. Viswanathan, a. Kruize, J. T. M. De Hosson, S.O'Dell, T. McKechnie, S. Rajagopalan, R. Vaidyanathan, and S.Seal, “Structure and property evaluation of a vacuum plasmasprayed nanostructured tungsten-hafnium carbide bulkcomposite,” Mater. Sci. Eng. A, vol. 477, no. 1-2, pp. 350-357,March 2008
Oxidation of the above mentioned materials as a tool is also a serious concern during FSW. FIG. 6 shows the high oxidative resistance of Tungsten and Rhenium at high temperature which is the key physical property in the development and the performance of the tool during the service. The FSW tool failures are mainly attributed to diffusion and wears. Ellingham diagram shown in FIG. 6 provides information about the regions of stability of oxide formation at high temperature. FIGS. 10A and 10B show oil hardened steel tool used in friction stir welding of A16061-20 vol % Al2O3 composite. It was noted that wear rate decreases (due to second phase hard particles) after the initial wear.
The challenges of finding a suitable tool material for friction stir welding of steels and high temperature alloys may be addressed by manipulating tungsten base alloys and composites. Although pure tungsten (W) has sufficient strength at elevated temperatures, its application has been limited due to very low toughness, particularly at room temperature, and also a large amount of wear when used as a tool material for FSW. Since tungsten is also susceptible to embrittlement and recrystallization at temperatures higher than 1200° C., Rhenium (Re) can be added to lower the ductile to brittle transition temperature and increases the recrystallization temperature. Furthermore, hafnium carbide particles may increase a microhardness of the tool material.
In view of the forgoing, one objective of the present invention is to provide a friction stir welding tool made of a composite of a tungsten-rhenium alloy, and hafnium carbide particles, wherein the hafnium carbide particles are homogenously dispersed within the tungsten-rhenium alloy, and which is fabricated by ball-milling a solid solution of tungsten-rhenium alloy and hafnium carbide particles followed by spark-plasma-sintering. Spark-plasma-sintering the solid solution forms a composite, wherein the tungsten-rhenium alloy has a crystallite size in the range of 20 to 100 nm. Another objective of the present invention relates to a method of friction stir welding a high strength metal joint using the friction stir welding tool.